9 min readConcept sheetSection A MCQ (1 mark) and passage parts; Section B very short answer (2 marks) on what electromagnetic induction is and how to make the induced current larger

Electromagnetic induction

A changing magnetic field through a coil makes a current flow in it: electromagnetic induction. Move the magnet faster or use more turns, and the current grows.

Discovered byMichael Faraday, in 1831

When current flowsOnly while the magnetic field through the coil is changing

Larger induced currentFaster change, a stronger magnet, more turns in the coil

Used inGenerators and transformers

Current from magnetism

Oersted showed that a current makes a magnetic field. Michael Faraday asked whether the reverse was possible. In 1831 he found that a changing magnetic field through a coil makes a current flow in it. This is electromagnetic induction, and the current is the induced current. The field through the coil can change in two ways. A magnet can move into or out of the coil, or the coil can move or turn near a magnet. The current flows only while the change is happening. Generators and transformers work on this principle.

Activity: current from a moving magnet and coil

You need a strong magnet, insulated copper wire, a cardboard tube and a galvanometer.

1
Wind many turns of insulated copper wire on a cardboard or bamboo tube. Join the two ends of the wire to a galvanometer (a meter that shows a small current).
2
Push a strong bar magnet quickly into the coil. Observe: the galvanometer needle deflects.
3
Hold the magnet still inside the coil. Observe: the needle returns to zero.
4
Pull the magnet out. Observe: the needle deflects the other way.
5
Move the magnet in and out faster. Observe: the deflection is bigger.
6
A simple version: wind 150 turns on a plastic syringe and join the ends to an LED. Slide a small magnet to and fro inside it.
The LED glows only while the magnet moves.

What makes the induced current larger

Moving the magnet or the coil faster, so the field changes more quickly.
Using a stronger magnet.
Using more turns of wire in the coil.
Changing the current in one coil can induce a current in a nearby coil, because its changing field passes through the second coil.

Fleming's right-hand rule (direction of the induced current)

Stretch the thumb, first finger and middle finger of the right hand at right angles to one another.
Point the thumb along the motion of the conductor and the first finger along the field.
The middle finger then gives the direction of the induced current.
Use the right hand for generators (motion makes current) and the left hand for motors (current makes motion).

Motor vs generator

Electric motorCurrent + magnetic field give motion. Changes electrical energy into mechanical energy. Direction by Fleming's left-hand rule.

Electric generatorMotion + magnetic field give current. Changes mechanical energy into electrical energy. Works on electromagnetic induction; direction by Fleming's right-hand rule.

Both have a coil turning between the poles of a magnet. What goes in and what comes out are swapped.

In real life

A bicycle dynamo lights its lamp only while the wheel turns, because the magnet inside must keep moving. Induction cooktops and wireless phone chargers also use changing magnetic fields to make currents flow.

Exam angle

1 marks: “Name the scientist who discovered electromagnetic induction.” Michael Faraday, in 1831.

2 marks: “A magnet is held still inside a coil joined to a galvanometer. Does the needle deflect? Give a reason.” No. A current is induced only while the magnetic field through the coil is changing. A still magnet gives an unchanging field.

3 marks: “What is electromagnetic induction? Describe an activity to show it, and give two ways to increase the induced current.” Making a current flow in a coil by changing the magnetic field through it. Coil joined to a galvanometer: pushing a magnet in or out deflects the needle; holding it still gives zero. Larger current: move the magnet faster, use more turns or a stronger magnet.

Remember it

No change, no current. Faster change, more turns, stronger magnet: more current.